Schistosome infections are studied quantitatively in man and in experimental animals. The number of adult worms present is related to passage of eggs in the excreta, accumulation of eggs in the tissues and the development of lesions in the tissues. Hepatic pathology produced by Schistosoma mansoni in Brazil and Egypt appears to be identical, both in the anatomic lesions produced and in the number of worms required to produce severe hepatic pathology. Schistosoma colonic polyposis is frequent in Egypt and extremely rare in Brazil in spite of comparable intensity of infection in these two countries. Obstructive uropathy in Egyptian patients examined at autopsy was related to the presence and intensity of S. haematobium infection; however, levels of infection were similar in cases of inconsequential and severe obstructive uropathy.